I. Field of the Invention
The present invention relates generally to communications systems in which data is transmitted by user communications devices. More particularly, the present invention is directed to a novel system and method for reducing interference generated by a code division multiple access (CDMA) communications device.
II. Description of the Related Art
The use of variable rate encoding techniques in a code division multiple access (CDMA) system provides the ability to reduce the amount of information used to represent speech or other data as compared with fixed rate encoding techniques. For example, by using variable rate vocoding techniques fewer bits of information may be transmitted when speech levels are low. In a CDMA communication system as described in the Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) Interim Standard IS-95A entitled xe2x80x9cMobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d variable rate encoded speech or data is transmitted by the communication unit using frames of a fixed time duration. For data rates which are less than the maximum rate, the extra space available in the frame may carry signaling bits, if available, or secondary traffic bits if the communication unit is utilizing, for example, simultaneous voice and data service options. However, if there are not enough bits between primary traffic and signaling or secondary traffic to fill up an entire frame, the remaining blank space in the frame may be distributed among the transmitted bits in a pseudo-random manner, resulting in xe2x80x9cburstsxe2x80x9d of data being transmitted intermittently throughout the frame. During this blank space, the transmitter is gated off.
Alternatively, for low rate encoded speech or other data, the information bits may be repeated as necessary to fill the frame instead of turning off the transmitter. In such a case, the power of the transmitter is nonetheless typically reduced to conserve battery life and system capacity, since the data redundancy in the frame allows a satisfactory probability of successful reception at a lower signal-to-noise ratio.
As can be seen, the use of variable rate transmission techniques leads to significant variation in the transmit power level of the CDMA transmitter during each frame. The variation in power level may be a mere reduction in the case of repeated bits, or turning off in the case of pseudo-random burst distribution throughout the frame. This variation of the subscriber station""s transmit power produces a transmitted signal which is capable of interfering with the operation of nearby electronic equipment. Because this variation modulates the amplitude of the RF energy transmitted, interference which is caused by such transmit power variation is referred to herein as xe2x80x9camplitude modulation interference,xe2x80x9d or simply xe2x80x9cAM interference.xe2x80x9d It has been found that many electronic devices, such as hearing aids, tend to act as AM detectors, leading to interference being incurred during operation in the presence of transmitters which vary the amplitude of the transmitted signal.
Accordingly, what is needed is a transmission scheme for a CDMA communication system which minimizes the potential for interference with electronic devices.
The present invention is directed to a system and method for reducing the interference experienced by sensitive electronic devices, such as hearing aids, operative in the vicinity of CDMA user communication devices. As mentioned previously, in CDMA communication systems the transmission of data at variable rates may yield a certain level of AM interference. The various embodiments of the present invention are designed to substantially reduce or eliminate such interference. The present invention is particularly suitable for implementation in a CDMA communication system, such as cellular, mobile, wireless local loop, PCS, and satellite applications, in which system users nominally transmit data at variable rates.
In the present invention, a reduced interference mode signal is provided to the controller of a CDMA communication device associated with an authorized hearing-impaired user. Such authorization will typically be extended to, for example, hearing-impaired users equipped with an electronic hearing aid or the like. In response to the reduced interference mode signal, a transmitter of the communications device is configured to transmit RF energy at a full-rate power level substantially continuously irrespective of the variable data rate at which the transmitter would otherwise be operative due either to the instantaneous level of user speech activity or to the data rate of non-voice data being provided to the transmitter.
The CDMA communication device may be configured for substantially continuous transmission in response to the constant transmission mode signal in many different ways. In a first embodiment of the present invention applicable to variable rate encoded speech, a variable rate vocoder within the transmitter is directed to encode voice data at a full-rate irrespective of the instantaneous level of user speech activity. By performing continuous full-rate vocoding, the variable rate vocoder generates sufficient bits to fill each frame, resulting in the continuous transmission of full-rate frames. By continuously transmitting full-rate frames, the communication device avoids the generation of the AM interference normally associated with the transmission of variable rate vocoded speech.
In a second embodiment of the present invention applicable to both variable rate encoded speech and non-speech data, a controller within the transmitter generates only full frames. When non-full rate data is provided to the controller, it generates full frames consisting of repeated versions of the non-full rate data. However, each of the frames containing repeated data is transmitted at a full rate power level. The transmit power level is not reduced in response to the amount of data redundancy in the frame. By continuously transmitting full frames at a full rate power level, the communication device avoids the generation of the AM interference normally associated with the transmission of variable rate vocoded speech and non-vocoded data.
In a third embodiment of the present invention applicable to both variable rate encoded speech and non-speech data, a controller in the communication device builds only full-rate frames of data for transmission. If there are not enough bits to fill the frame between primary traffic, signaling and secondary traffic, the controller pads the frame with signaling traffic of a unique pattern to fill the full rate frame. In this embodiment, the base station recognizes the unique pattern of signaling traffic, and discards it upon reception. By continuously transmitting full-rate frames, the communication device avoids the generation of the AM interference normally associated with the transmission of variable rate vocoded speech and non-vocoded data.
It should be noted that in each of the above-described embodiments, the present invention is not limited to a particular frame length or multiplex format. For example, the teachings of the present invention are equally applicable to the 14.4 kbps service option multiplex format of ANSI J-STD-008 entitled xe2x80x9cPersonal Stationxe2x80x94Base Station Compatibility Requirements for 1.8 to 2.0 Ghz Code Division Multiple Access (CDMA) Personal Communications Systemsxe2x80x9d as they are to the 9.6 kbps service option multiplex format of IS-95A.
A number of authorization schemes may be instituted as a means of ensuring that only those CDMA communication units associated with selected eligible users (e.g., hearing impaired users) are permitted to engage in the type of reduced interference transmission contemplated by the invention. For example, only eligible hearing impaired users could be allowed to purchase CDMA communication units pre-authorized for operation in a reduced interference mode. Alternately, eligible hearing impaired users could obtain authorization from their cellular service provider after purchase of a CDMA communication unit capable of reduced interference mode operation. Additionally, reduced interference mode operation, once authorized, could be activated either by dataport programming of the communication device or by over-the-air service programming.